1. Field of the Invention
The present invention relates generally to improvements in optical communication systems. More particularly, the present invention relates to four-photon mixing techniques for frequency converting and/or phase conjugating optical signals.
2. Description of Prior Art
In optical communication systems, it is often desirable to four-photon mix an optical signal. For example, one important aspect of four-photon mixing is that it can produce the phase conjugate of an optical signal. Phase conjugating an optical signal at the midpoint of an optical fiber span can compensate for fiber chromatic dispersion and thus allow the span to support higher bit rate-distance products. Chromatic dispersion refers to a phenomenon in which the speed of an optical signal through the fiber varies as a function of the optical signal frequency or wavelength. Because the phase conjugate of a given signal exhibits a reversal of phase as a function of time, mid-span conjugation allows the effects of dispersion in the first half of a fiber span to be canceled as the phase conjugated signal propagates along the second half. See A. Yariv, D. Fekete and D. Pepper, "Compensation for channel dispersion by nonlinear optical phase conjugation", Optics Letters, vol 4, pp 52-54, 1979. Midsystem optical phase conjugation has extended the bit rate-distance product achievable over the normal dispersion fiber which makes up much of the world's existing fiber communication channels. See A. Gnauck, R. Jopson and R. Derosier, "10 Gb/s 360 km Transmission over Dispersive Fiber Using Midsystem Spectral Inversion", IEEE Photonics Technology Letters, vol 5, no 6, June 1993. Frequency conversion is used in many other applications, including compensation for stimulated Raman scattering in multi-channel systems by global inversion of the channel signal frequencies, routing in wavelength division multiplexed (WDM) networks and switching in wavelength-division switches.
Frequency conversion of optical signals is typically performed using four-photon mixing, also commonly referred to as four-wave mixing. Four-photon mixing is a nonlinear process which produces various mixing products by mixing an input optical communication signal with either one or two higher power optical signals, or pumps, in a nonlinear mixing medium such as a semiconductor laser, a semiconductor laser amplifier or a length of dispersion-shifted fiber. However, the efficiency of the four-photon mixing process is highly dependent upon the relative polarizations of the input optical signal and the pump. Since the input signal polarization typically varies randomly with time, to maintain optimal efficiency in a four-photon mixing process one usually must control or adjust for the variation. Available techniques for controlling relative signal and pump polarization typically utilize polarization controllers or other manual or automated polarization adjustment hardware. Other disadvantages of existing polarization control include signal attenuation and limited optical bandwidth in some techniques.
Failure to maintain proper polarization alignment between the signal and the pump may result in a substantial decrease in the power of the output mixing products. In fact, for some polarization combinations, the power in the mixing products can be effectively zero. When four-photon mixing is used to phase conjugate an input signal, the advantages of optical phase conjugation would often be more than offset by a reduction in conjugated signal power. Since detecting and adjusting relative signal polarizations requires additional components, equipment and expense, polarization sensitivity presently limits the usefulness of frequency conversion and phase conjugation in commercial applications.
A recently developed experimental technique attempts to alleviate polarization sensitivity in four-photon mixing by using a polarization beam splitter and a fiber loop to produce and mix separately polarized versions of both the incoming optical signal and the pump. See T. Hasegawa et al., "Multi-Channel Frequency Conversion Over 1 THz Using Fiber Four-Wave Mixing", Post Deadline Digest of the Optical Amplifiers and their Applications Conference, paper PD-7, Jul. 4-6, 1993, Yokohama, Japan. Although the Hasegawa fiber loop four-photon mixing technique apparently reduces the sensitivity of the mixing process efficiency to incoming signal polarization, it suffers from a number of significant drawbacks. For example, a polarization controller is required in the fiber loop in order to effectuate the proper recombination of the different polarizations of the mixing products. This leads to additional hardware costs both for the polarization controller itself as well as for any additional devices required to appropriately adjust the polarization controller. Furthermore, the fiber loop uses relatively long lengths of dispersion-shifted or non-dispersive fiber to serve as a nonlinear medium for four-photon mixing. As such, the fiber loop technique may not be readily applicable to other commonly used nonlinear four-photon mixing media, such as semiconductor laser amplifiers. Since the technique depends on the polarization-independent nonlinear properties of optical fiber and since it requires that the signal loop back to the polarization splitter, it would be difficult to implement in a commercially advantageous form such as a photonic integrated circuit. Another disadvantage of this technique is signal attenuation in the polarization splitter.
In many currently available optical mixing processes, the pump signal power is limited by an effect known as stimulated Brillouin scattering (SBS). SBS distortion usually becomes a significant factor in fibers for pump powers in the range of 3.0 to 10.0 dBm, where Dbm refers to decibels relative to one milliwatt. As is well known, the SBS threshold is raised when the linewidth of the pump signal is wider than the SBS bandwidth, which is approximately 20 Mhz. The linewidth of the pump signal may be artificially widened beyond the SBS bandwidth by phase modulating the pump signal. However, using phase modulation to raise the SBS threshold also broadens the linewidths of the resulting mixing products, which, after the mixing product passes through chromatic dispersion, may result in phase distortion at the signal receiver and degradation in system performance. Current techniques typically cannot provide both an increase in the SBS threshold using phase modulation, and a narrow linewidth output mixing product.
As is apparent from the above, a need exists for a polarization-insensitive optical mixer which produces relatively constant frequency converted and/or phase conjugated signal output power regardless of input signal polarization. Maximum benefit will thereby be obtained in systems compensating dispersion by optical phase conjugation, as well as in other frequency conversion applications. The optical four-photon mixer should be useful with any type of nonlinear mixing device, and therefore suitable for implementation in the form of a photonic integrated circuit. Furthermore, the optical mixer should be capable of using low frequency phase modulation to raise pump signal SBS thresholds without causing phase distortion in the output mixing products.